Until recently two approaches have been used to suppress transients in electronic circuits. The first is a Zener diode type of device, that is, an electronic device which is strongly non-linear but which is without discontinuity in the current/voltage characteristic. These devices are positioned so as to hold off current flow during the circuit's usual operating conditions but allow current flow during conditions of excessive voltage application due to their "avalanche" current/voltage characteristic.
A second type of device used for this application in the past has been the gas ionization or "spark gap" device. These devices are characterized by a continuous current-voltage curve with a negative resistance region. In a basic configuration they are comprised of two electrodes separated by some small distance in a sealed container filled with inert gas. Under normal operating conditions the voltage across the device is insufficient to ionize the gas and therefore there is no current flow. Ionization of the gas is a function of the magnitude and duration of the applied voltage. At some low voltage limit the gas will not ionize regardless of the duration of application. As the voltage is increased the duration of the pulse necessary to ionize the gas is decreased.
A far more recently developed device which may be used for transient supression is the metal oxide switching component developed at Yeshiva University by Paul M. Raccah, Teodoro Halpern and Soo Hee Shim (U.S. patent application Ser. No. 529,222). This device consists of a layer of polycrystalline refractory transistion metal oxide, such as NbO.sub.2, grown on a conducting refractory substrate. This device has the characteristic of providing a high resistance device when subjected to a high voltage pulse. The device is capable of handling currents in the order of 80 amperes while maintaining a response time shorter than 0.7 nanoseconds.
All three of the above described devices when used alone have undesirable limitations as protective devices. The response time of the gas ionization device is in the order of 3 to 4 nanoseconds, for pulses in the order of 2000 volts and increases rapidly as voltage is reduced, being in the order of milliseconds for pulses of 100 volt amplitude.
The Zener diode has excellent response time but is unable to combine high power capabilities with low insertion loss.
The metal oxide switching component provides excellent power handling capability with good response time but fails to reduce the voltage sufficiently for many applications, thereby imperilling the ciruitry it was intended to protect.
When these existing devices are combined in the special manner described herein an electromagnetic pulse suppressor is obtainable which has rapid response time, excellent power handling capabilities and which will protect all but the most sensitive circuitry.